To make the job easier the first thing you will need is some help...lifting 12ft beams is a two man job. The way we constructed the frame was to secure the uprights as best as possible, this was done by putting the upright beam in place and then securing rocks around the base so the beam didnt move. Dont be afraid to hammer those rocks in and chock the base of the beam.
Next the beam ends were placed and secured into the holes made in the cave facia - again made secure with rock, at this stage some adjustment may need to be made to get the beams level and all angles as near as they can be. Once in and secure place the other end of the beam on top of the upright.
To create strength and balance to the "uncemented" frame the cross bar comes next - once in place this will make the frame more secure. After some final adjustments its time for cement in the holes in the facia and for around the base of the uprights. Use plenty of rocks (some quite small) and flick the cement mix right into the hole around the beam end. For the base of the uprights hammer some 4-5" nails in - this will give the base some extra purchase once submerged in the cement. If you have any lengths of plywood or old pallet wood it is good to brace the beams to prevent movement.
For pergola beams that cannot be sunk into the ground there is an alternative. We sat ours directly onto the concrete patio, placed a ring of stones around the base and backfilled the space with rock and concrete. The cement was 6 inches deep and needs 48 hours to set. Again, hammer nails into the base of the pole around its circumference before pouring the cement. Douse the void with water first to obtain a better purchase.
The size and weight of the beams act as an advantage to the strength of the frame - its a big, heavy construction and secured properly in place wont be going anywhere in a hurry.
Although the beams are probably decades old and have been treated in the past they will need treating again. Deisel is commonly used to eradicate woodworm and a nice wood stain can re-vamp the whole construction.
Next up: Making sure it will last, fitting the "roof" and the total costs involved.